Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
An ICD is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. ICDs include, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device.
An ICD may be programmed to sense a tachyarrhythmia and to deliver an escalating series of pulse therapies in an effort to correct this arrhythmia. For example, U.S. Pat. No. 5,458,619 to Olson shows a device that begins charging high voltage capacitors on detection of an arrhythmia. During the charging period, the device delivers a series of antitachycardia (ATC) pacing pulses. The number of pulses may be varied as a function of the voltage to which the capacitors are to be charged, so that more extended therapies may be available where allowed by longer charging times. After the ATC pulses, the device evaluates the heart rhythm to determine whether the tachyarrhythmia has terminated. If not, when the capacitor has charged, a high voltage cardioversion or defibrillation pulse is delivered.
ICDs, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors typically consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion- producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388. Conventional capacitor cases using metallic cases are generally known, such as those disclosed in U.S. Pat. No. 5,522,851 issued to Fayram.
Aluminum electrolytic capacitors tend to degrade with time. This is due, in part, to water in the electrolyte attacking the thin film of aluminum oxide (Al2O3) formed on the anode surface. Deformation of the aluminum oxide increases the leakage current of the capacitor, such that when one or more capacitors are used for shock delivery in an ICD, the first shock (after a hiatus) will have a significantly longer charge time. Unfortunately, one cannot remove all of the water from the electrolyte, as it is needed for conduction, as well as for the formation of the cut edges of the aluminum foil after assembly. Therefore, what is needed in the art is a method of reducing the water content within an electrolytic capacitor casing to reduce degradation of the capacitor over time.